1. Field of the Invention
The present invention relates to a process for continuously producing a pitch-based carbon fiber felt excellent in uniformity of the unit weight and physical properties. The term "carbon fiber felt" as used herein includes activated carbon fiber felt as well.
Specifically, the pitch-based carbon fiber felt which is produced by the process of the present invention is excellent in uniformity of the unit weight and physical properties and provides high-performance thermal insulator, cushioning thermal insulator, filter media, adsorbent and the like. In particular, the pitch-based carbon fiber felt which is produced from optically anisotropic pitch as the raw meterial can be used in carbon-carbon composite, electrodes of an electric cell, nuclear fusion reactor walls, etc. Moreover, the activated carbon fiber felt can be efficiently utilized in water purification, solvent recovery and the like.
2. Description of Related Art
A pitch-based carbon fiber felt has heretofore been produced by the process comprising the step of collecting the pitch fibers that have been spun out by centrifugal spinning system, vortical spinning system or spun-bond spinning system in the form of tow or sheet on a perforated belt; the step of stabilizing treatment in an oxidative atmosphere; the step of carbonizing treatment in an atmosphere of an inert gas, the step of direct activating treatment in an atmosphere of an activating gas, or the steps of carbonizing treatment in an atmosphere of an inert gas and subsequent activating treatment of the carbonized fibers; the step of webbing the pitch-based carbon fiber precursor in the form of tow or sheet obtained through the foregoing steps via independent carding treatment; the step of laminating the webbed precursor; and the step of fixing the fibers by entangling the fibers by needle punching, water jet and the like, or by bonding the fibers with an adhesive.
In the above-mentioned process, the fiber precursor in the form of tow or sheet brings about generally a shrinkage of about 5 to 20% in carbonizing treatment and about 10 to 50% in the activating treatment due to an intrinsic shrinkage caused by weight loss thereof and flexure of the fibers in the carbonizing and activating treatment.
The remarkable shrinkage of the fibers gives rise to ununiform shrinkage thereof in a carbonization or activation furnace which will lead to ununiform unit weight of the tow or sheet obtained therethrough, and in the extreme case, to breakage of the tow or sheet. Particularly in the case of activated carbon fiber, the above-mentioned shrinkage leads to ununiform specific surface area.
Also the above-mentioned process involves the problems of a lower process yield and impossibility of enhancing its strength because of the fiber precursor in the form of tow or sheet being hackled in the course of carding treatment. Particularly, in the case of low elongation fibers such as optically anisotropic pitch-based carbon fiber or particularly low-strength fibers such as activated carbon fiber, the above-mentioned process makes it difficult to produce a felt containing 100% of pitch-based carbon fiber and having a uniform unit weight, sufficient handleability and high strength.
On the other hand, melt-blow spinning system has the advantage of favorable productivity and capability of producing fine fibers having a fiber diameter of about 10 .mu.m or smaller. However, in the case where the pitch-based fibers obtained by melt-blow spinning system having a finite length, especially the fine fibers having an average fiber diameter of about 10 .mu.m or smaller are applied to the process for producing the pitch-based carbon fiber felt, there are caused more frequently the aforesaid shrinkage and/or breakage of the tow or sheet during the carbonizing treatment or activating treatment and the breakage of the fibers in the carding treatment, showing the tendency of increased ununiformity of the physical properties of the obtained felt such as the unit weight and specific surface area.
Conclusively, it was impossible by any of the conventional processes to produce a felt excellent in uniformity of physical properties in a high yield from pitch-based carbon fiber or activated carbon fiber. As a result of intensive investigation made by the present inventors on the above-mentioned problems, it was found by them to be effective to carry out the carbonizing treatment or activating treatment of the carbon fiber precursor which brings about a remarkable shrinkage under the condition enabling free shrinkage of the web. The present invention has been accomplished on the basis of the aforestated finding and information.